Conventional internal combustion engines employ piston and cylinder arrangements that tend to vibrate during operation. The vibration often creates a disturbance in a vehicle passenger compartment and is considered undesirable.
Most internal combustion engines develop power in impulses generated by the explosion of a combination of air and fuel in the engine's cylinders. The power is transferred to pistons that are located in the cylinders and are coupled to a rotating crankshaft with connecting rods. The power then flows to a flywheel that is connected to other downstream components of a powertrain. All conventional, single crankshaft, piston engines have a firing frequency vibration caused by uneven torque delivery to the flywheel. On a combustion or expansion event, the flywheel's rotational speed increases and, on a compression event, the rotational speed of the flywheel decreases. The torque that causes vibrational speed variations of the flywheel reacts against the cylinder block and causes torsional vibration of the cylinder block.
This fluctuating torque causes one source of vibration. Other disturbing engine vibrations are caused by unbalanced accelerations of internal engine components, especially linear accelerations of the piston masses within the cylinder bores. To address these problems, rubber engine mounts have been used to isolate the vehicle chassis from much of the cylinder block vibration. Still, some vibration is transmitted through the mounts and is sensed in the passenger compartment.
A partial solution is to have multi-cylinder engines generally configured so that the linear acceleration forces of the various pistons partially or completely cancel each other. Inline and opposed 6-cylinder engines, as well as inline and 90 degree V8 engines, usually have theoretically perfect balance of piston acceleration forces, but most other engines have residual unbalanced forces or moments. For example, all single crankshaft V6 engines with less than 180 degrees of bank angle have inherent unbalanced couples due to piston acceleration forces. Furthermore, all conventional single crankshaft engines have unbalanced torsional accelerations imposed upon the block structure due to flywheel rotational accelerations.
As an example, the Volkswagen 15 degree bank angle V6 engine is narrower than other 60 or 90 degree V6 engines and has a one-piece cylinder head that spans between two cylinder banks. However, there are numerous undesirable qualities with such a design. The intake manifold is on one side of the cylinder head and the exhaust manifold is on the other, causing three cylinders to have long intake and short exhaust passages while the other three cylinders have short intake and long exhaust passages. An asymmetry exists between the cylinder banks with regard to the location of the intake and exhaust valves. Further, one bank has mostly vertical intake valves and highly inclined exhaust valves, while the other bank has highly inclined intake valves and mostly vertical exhaust valves. Finally, the center planes of the cylinder bores intersect some distance below the crankshaft rotational axis, so that the cylinder bores on each bank are offset from the crankshaft axis in opposite directions. This arrangement causes the piston velocities in each of the two banks to be different. On one bank, the pistons are slower during upward motion than they are during downward motion. On the other bank, the pistons are faster during upward motion than they are during downward motion.
The use of two crankshafts in one cylinder block is not unprecedented. One example can be found in the Ariel motorcycle. The Ariel motorcycle was manufactured for many years with a dual crankshaft engine. This Ariel “Square Four” engine included two inline, two-cylinder crankshafts operating in a common cylinder block structure, with the resulting four cylinder bores being oriented in a square fashion. Each of the two crankshafts operates two pistons, with a 180-degree phase angle between the crankpins on each crankshaft. One pair of straight cut spur gears is arranged to couple the crankshafts to each other to make the crankshafts rotate in opposite directions. This arrangement has some apparent drawbacks. First, the arrangement is very noisy in operation because the single gearset has backlash and rattles each time the direction of torque transfer reverses. Also, because each cylinder bank contains only two cylinders, each bank of cylinders has a second order vertical shaking force that is in phase with the vertical shake of the other bank. Thus, the whole engine assembly has a second order vertical shake equivalent to that of an inline four cylinder engine. Furthermore, the two counter-rotating crankshafts do not carry equal amounts of rotating inertia so the firing pulse accelerations of the crankshafts produce a reaction on the engine's cylinder block.
Based on the above, there is a need in the art for a dual crankshaft engine that is well balanced and produces much less vibration than conventional engines, while avoiding the disadvantages set forth above.